1. Field of the Invention
The present invention relates to a stratified scavenging two-stroke engine, in more specifically, relates to a stratified scavenging two-stroke engine provided with an air passage for supplying air for pre-scavenging into a scavenging passage and a check valve for controlling open and close of the air passage to the scavenging passage.
2. Description of Related Art
In a stratified two-stroke engine, during an upward stroke of a piston, a gaseous mixture is supplied from an intake passage to the inside of a crankcase by a negative pressure generated inside of the crankcase, and air is supplied from an air passage into a scavenging passage. Furthermore, during a downward stroke of the piston, prior to supply of the gaseous mixture inside of the crankcase, the air, that has been supplied into the scavenging passage during the upward stroke of the piston, is supplied to the inside of the cylinder as air for pre-scavenging. Consequently, since a layer of air is present between a combustion gas (exhaust gas) produced by combustion and a gaseous mixture that is newly supplied via the scavenging passage, it is possible to prevent the gaseous mixture from being mixed into the combustion gas, and to thereby prevent blow-by of unburned gas through an exhaust port.
Here, such stratified two-stroke engines are roughly categorized into the following two types according to the method for restricting air flow from the air passage to the scavenging passage.
In one of the categorized stratified two-stroke engines, a check valve is provided in an air passage, which permits a flow of air from the air passage toward the scavenging passage and inhibits a flow of air and the gaseous mixture in the opposite direction, that is, a flow from the scavenging passage through the air passage toward the outside.
In the other of the categorized stratified two-stroke engines, a groove is formed on a side surface of a piston, through which the scavenging passage temporarily communicates with the air passage to supply air into the scavenging passage, and the air passage is closed by the piston at a time of supplying the gaseous mixture to the inside of the cylinder.
WO 2010/035684 discloses a stratified two-stroke engine of the former type.
In the stratified two-stroke engine disclosed in the above document, to a scavenging passage extending from a scavenging intake that opens to the inside of a crankcase to a scavenging port that opens to the inside of a cylinder, an air passage for supplying air for pre-scavenging is connected at a substantially intermediate position between the scavenging intake and the scavenging port. Furthermore, in the connecting portion of the air passage with the scavenging passage, there is provided a check valve (for example, a reed valve) which inhibits a reverse flow from the scavenging passage to the air passage. The engine has a piston having a lower surface into which a cutout is formed so that the scavenging port opens through the cutout in a transition period from a late stage of an upward stroke to an initial stage of a downward stroke. Accordingly, a negative pressure generated inside of the crankcase propagates through the scavenging port into the scavenging passage, so that air for pre-scavenging is supplied from the air passage to the scavenging passage. Here, in order to prevent the negative pressure inside of the crankcase from propagating into the scavenging passage through the scavenging intake, the scavenging intake is provided with a check valve (for example, a reed valve) which inhibits a flow of air from the scavenging passage to the inside of the crankcase.
According to such a construction, the air, that has been supplied into the scavenging passage, flows through the scavenging passage in one direction toward the scavenging port without flowing from the connecting portion with the air passage toward the scavenging intake. Accordingly, since mixing of a gaseous mixture into the air for pre-scavenging is suppressed, stratified separation between the air and the gaseous mixture is maintained, and blow-by of unburned gas through an exhaust port is thereby prevented.